Successfully delivering intensive yet safe gait therapy to individuals with significant walking deficits presents the greatest challenges to even the most skilled therapists. In the acute stages of many neurological injuries such as stroke, spinal cord injury, or traumatic brain injury, individuals often exhibit highly unstable walking patterns and poor endurance, making it difficult to safely practice gait for both the patient and therapist. Because of this, there has been a big push in rehabilitation centers to move over-ground gait training to the treadmill where body-weight support systems can help minimize falls while at the same time raising the intensity of the training. Numerous studies have investigated the effectiveness of body-weight supported treadmill training and have found that this mode of gait training promotes gains in walking ability similar to or greater than conventional gait training. Unfortunately there is a gap in technologies on the market for transitioning subjects from training on a treadmill to safe, weight-supported over-ground gait training. Practicing walking over-ground is critical, as our recent studies have demonstrated key differences between walking on a treadmill and walking over-ground. Since a primary goal of all individuals with walking impairments is to walk in their homes and in the community, it is imperative that therapeutic interventions involve over-ground gait training. Over the last three years, we have been developing an over-ground body-weight support gait training system called ZeroG, which provides individuals with gait impairments dynamic unloading of a percentage of their body-weight as they practice walking over-ground. The system, which rides along an overhead rail, provides both static and dynamic unloading, and has an active trolley that automatically follows the subject as they walk. Here, a small motor drives the system so that it stays directly above the subject as they walk over-ground. While the existing trolley tracking system works well for over-ground gait in individuals with slow walking speeds, it is limited in speed as well as the amount of force it can provide. Adding additional force to the trolley is important for controlling the forward momentum of the subject during falls and for stabilization when lifting the subject from their wheelchair or off the floor. In addition, our long-term goals are to add a sit-to-stand protocols as well as dynamic balance tasks, which will require additional force and better performance from the trolley. The goal of this Phase I SBIR is to retrofit the ZeroG trolley with a larger motor and to develop enhanced control algorithms that will improve subject tracking during over-ground gait and can be used for advanced training activities. The hardware and controllers developed in Phase I will then be utilized in the development of advanced postural and dynamic tasks in Phase II. Enhancing the capabilities of ZeroG throughout Phases I &II of this SBIR will allow individuals with lower limb impairments the ability to practice over-ground gait, as well as postural and dynamic balance tasks, making it one of the most versatile gait training systems in the world. As a result, we believe that individuals who train with ZeroG will experience enhanced gains in over-ground walking ability when compared to existing conventional rehabilitation strategies as ZeroG will allow therapists to train patients through a broad spectrum of activities early after neurological injuries in a safe, controlled manner. This system can be used in the functional rehabilitation of various disabling conditions, including stroke and spinal cord injury to amputations and with elderly individuals. In addition to clinical benefits, a body-weight support system of this nature can also be used to foster new research studies that focus on gait impairments. PUBLIC HEALTH RELEVANCE: Recent studies have reported that in the United States alone, more than 18% of the population (or equivalently 35.6 million individuals) suffer from some form of walking disorder that negatively impacts their ability to ambulate at a community level (DeJong et al., 2002). Walking disorders, defined as deficits in balance, stability and endurance while unsupported, are common to individuals following stroke, multiple sclerosis (MS), cerebral palsy (CP), spinal cord injury (SCI), amputation, and normal complications related to general aging. Unfortunately, the number of people who have trouble ambulating will continue to worsen in upcoming years due to our aging population. The US Census Bureau projects that the size of the elderly population (those 65 years or older) will rise from approximately 33+ million (12.7% of the US population in 1999) to 53 million in 2020 and 77 million by 2040 (U.S. Census Bureau, 2001). This trend raises significant concerns since over half of the elderly (36.3%) have some form of balance and gait impairment while approximately 17.9% have significant walking disorders (DeJong, 2002). Furthermore, the National Stroke Association has projected that in the next 20 years, the number of US citizens who suffer a stroke will surpass 1 million per year, where half of these stroke survivors will experience long-term disabilities including gait impairments (Stroke Association, 2008). Why is this an important health care issue? Deficits in balance control and walking disorders often result in individuals not feeling secure outside of their immediate environment, keeping them from truly re-integrating into society and from full social participation. Unfortunately treating these disorders has been severely hampered by economic pressures placed on rehabilitation centers by health-care providers. Today, rehabilitation centers have less time to treat their patients than ever before, where the median length of stay in inpatient rehabilitation therapy has decreased from 20 to 12 days between 1994 and 2001, while the proportional decreasing length of stay was greatest for orthopedic patients (Ottenbacheret al., 2004). This often results in rehabilitation centers being forced to discharge patients even though they are continuing to make significant gains in walking ability. The troubling paradox with this statistic is that extensive therapy after many neurological and musculoskeletal pathologies leads to significant returns in balance control and walking ability (Barbeauet al, 1998.). These improvements often allow these individuals to independently perform ADLs and in some cases, return to work. It is therefore imperative for therapists to have the appropriate tools to treat gait disorders in highly impaired subjects early after their injuries and with appropriate intensity. The system that will be further developed and tested in this SBIR will help address walking disorders across numerous subject populations, allowing therapists and clinicians to safely treat patients early in the stages of recovery, a factor known to be important to influencing gait outcomes (Horn et al., 2005).